Process and system of desalination by direct contact heat transfer

ABSTRACT

A process and system for obtaining fresh water from salt water such as sea water, by direct contact heat transfer using staged evaporator-condenser units operating with a low boiling water-immiscible liquid hydrocarbon. Each stage comprises a closed vessel divided into two compartments, an evaporator and a condenser, by a bubble cap tray similar in construction to those used in distillation. Relatively hot fresh water is introduced into the liquid hydrocarbon contained in the lower compartment of each stage, causing boiling of the hydrocarbon at a temperature differnce of 2°-4° F. Hydrocarbon vapors pass from the evaporator compartment through the bubble caps to the upper condenser compartment where they are condensed in contact with a colder saline stream, and the condensed hydrocarbon is returned to the lower compartment. The heated saline stream is flash evaporated and the vapors are condensed in direct contact with recycled fresh water from the evaporator compartments, and a portion of this combined stream is taken off as product, the remainder of the combined stream being further heated and reintroduced into the evaporator compartments of the staged units. Sea water feed is introduced into the condenser compartment of the last stage and passes into the respective condenser compartments of each of the preceding stages, and is further heated therein in each stage, and the hot fresh water introduced into the evaporator compartment of the first stage is passed through the evaporator compartments of each succssive stage and is further cooled in each successive stage.

This is a division, of application Ser. No. 907,143 filed May 17, 1978now U.S. Pat. No. 4,238,296.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an efficient process for desalination of saltwater, particularly sea water, and production of fresh water. Theinvention is particularly concerned with a process and system of theabove type for recovery of fresh water from salt water or sea water bydirect contact heat transfer between hot water and an immiscible liquid,particularly a liquid hydrocarbon of low vapor pressure, to causeboiling thereof, and effecting direct contact heat transfer between thevapor of such immiscible liquid and the salt water to condense suchvapor and heat the salt water, employing an arrangement of stagedevaporator-condenser units for carrying out such boiling andcondensation operations, and utilizing the heated salt water togeneraate fresh water.

2. Prior Art

Desalination is a growing industry in many parts of the world. Not onlythe countries with vast areas of arid lands, but the developed and thedeveloping countries also are increasingly producing fresh water bydesalination to meet the demands of growing population and risingstandards of living.

Multi-stage flash and multi-stage evaporation are the most importantprocesses currently in use for desalination of sea water. Theseprocesses suffer from two major disadvantages. In the first place, bothrequire large metallic heat transfer surfaces. The cost of the heattransfer surface for these processes is about 35 to 40% of the totalcapital investment. Also, the corrosion and scaling of these surfacesare difficult to avoid, thus further increasing the cost by the need forreplacement of corroded metallic surfaces. Secondly, the cost of energyrequirements for these processes is relatively large of the order ofabout $2.00 per 1,000 gallons of fresh water produced. Thus, anydesalination process which eliminates, partly or wholly, the need formetallic heat transfer surface and/or requires less energy or a lowerquality energy has attractive advantages.

A number of other methods and systems are also in use or being developedfor desalination. These latter processes are based on the principles ofvapor compression, reverse osmosis, freeze crystallization and ionexchange. All of these latter processes are relatively less attractive.

The improved processes described in the Smith U.S. Pat. Nos. 3,640,850and 3,856,631 are based on heat transfer with direct contact betweenimmiscible fluids. Hence these systems do not require metallic heattransfer surfaces, and such processes can operate with smaller amountsof energy per unit of water produced and with relatively low qualityheat.

In the Smith patents hot sea water is flashed in a flash chamber and thewater vapor generated is condensed in direct contact with fresh water.The hot fresh water is now brought in contact with a hydrocarbon liquid,which is immiscible with water. The hydrocarbon evaporates and the vaporis condensed in contact with sea water which is heated due to transportof latent heat released from the hydrocarbon vapor, and the heated seawater is heated further by an external heat exchanger. The hot sea waterthen enters the flash chamber. Although the use of metallic heattransfer surfaces thus is virtually eliminated, this design involves theflow of hydrocarbon and water in opposing directions and in contact witheach other, the hydrocarbon following a substantially horizontal flowpath between a plurality of evaporator and condenser units.

Other related but less pertinent prior art is set forth below.

The El-Roy Pat. U.S. Pat. No. 3,337,421 shows a multi-stage system inwhich vaporized hydrocarbon is condensed by direct contact with a salinestream. However, the hydrocarbon is vaporized by indirect heat exchange.

Guptill et al, in U.S. Pat. No. 3,392,089 discloses liquid-liquid heatexchange between a hot fresh water stream or condensate from a multipleeffect evaporator, and a hydrocarbon stream. The heated hydrocarbon isnot vaporized as it is chosen to have a high vapor pressure, and is usedto transfer heat to a saline stream, by liquid-liquid heat exchange, andthe preheated saline stream is fed to the multiple effect evaporator.

U.S. Pat. No. 3,446,711 discloses the condensation of steam by directcontact with a colder liquid hydrocarbon. The heated hydrocarbon is thenpassed in liquid-liquid exchange with a cold saline feed stream.

Woodward in U.S. Pat. No. 3,219,554 discloses liquid-liquid heatexchange between a hot fresh water stream and a hydrocarbon and betweenthe heated hydrocarbon and an incoming saline stream. The hydrocarbonhas a sufficiently high vapor pressure to preclude any significantvaporization.

U.S. Pat. No. 3,232,847 to Hoff employs a high boiling hydrocarbon whichis passed counter current in liquid-liquid exchange to brine in aheating section and is used as a direct contact condensing medium forsteam in a second section.

Osdor, U.S. Pat. No. 3,741,878 discloses a similar system in that a lowvapor pressure hydrocarbon is used as a heat exchange medium.

It is an object of the present invention to provide an improved processand system for desalination of salt water or sea water, employing directcontact heat exchange. A further object is the provision of a processand system of the foregoing type, employing liquid-liquid direct contactheat exchange between an immiscible liquid and hot water to vaporizesuch liquid, and passage of such vapor into direct contact with the saltwater to condense the vapor and transfer heat to the salt water. Aparticular object is to provide a process and system of theaforementioned type having high heat transfer coefficients due tointimate mixing between the two liquid phases, to promote boiling at lowtemperature differential between water and immiscible liquid orhydrocarbon, and hence improved thermal efficiency, and greater economydue to utilization of low quality energy and optimum selection ofpressures in the staged system.

SUMMARY OF THE INVENTION

The invention process and system of desalination employs the basicprinciple of transferring heat from hot fresh water to sea water by theprocess of evaporation of a liquid which is immiscible with water,preferably a hydrocarbon liquid, followed by condensation of the vapor,both the evaporation and condensation being carried out in directcontact with water. Thus, the evaporation of the immiscible orhydrocarbon liquid is carried out by contacting hot fresh water withsuch liquid, and the condensation of the hydrocarbon vapor is carriedout by contacting such vapor with sea water, to thereby heat the seawater.

The operating conditions are maintained so as to insure maximum boilingof immiscible liquid or hydrocarbon in contact with the hot water at alow temperature differential, particularly of the order of about 2°-4°F., between these two phases, and to obtain large volumetric heattransfer coefficients ranging from about 67,000 to 156,000 Btu//ft³ (hr)(°F.), by direct contact between the immiscible phases. Theseadvantageous conditions substantially reduce the sizes of the equipmentrequired for a given rated capacity of desalination, and increase theoperating efficiency by reducing the amount of external energy requiredfor operation of the process.

In the present invention, a plurality of condenser-evaporator units areconnected together in a staged operation. Each condenser-evaporator unithas two sections, one for evaporation and the other for condensation ofimmiscible liquid or hydrocarbon. The two sections are separated by anumber of bubble caps. Thus, hot fresh water is inroduced, preferably inthe form of a jet, into the lower evaporator section of each unit,causing evaporation of the low boiling immiscible liquid or hydrocarbonand the resulting vapor passes through the bubble caps into directcontact with sea water in the upper condenser section of each unit,causing condensation of the vapor and heating of the sea water. Thecondensed immiscible liquid in the condenser section is then returned orrecycled by gravity from the upper condenser section to the lowerevaporator section.

The hot fresh water is inroduced into the evaporator section of thefirst stage, and then successively into the evaporator sections of eachsubsequent stage to the last stage, the temperature of the hot waterbeing reduced successively in each of the stages. The sea water feed isintroduced first into the condenser section of the last stage of theprocess and passes successively to the condenser sections of eachprevious stage to the first stage, the temperature of the sea waterincreasing in each successive stage to the first stage.

Thus, the immiscible liquid or hydrocarbon medium remains static orstationary in each stage, that is, such liquid does not flow from onestage to the next as do the streams of hot fresh water and sea water, asdistinguished from the process and system of the above noted Smithpatents. Further, with hydrocarbon mixtures of appropriate compositions,the pressure in all stages can be maintained substantially equal eventhough the temperature varies from stage to stage. Equal pressures inthe respective stages results in additional economic advantages.

The immiscible liquid medium is a low vapor pressure liquid having aboiling point lower than water. Preferably such immiscible liquid is anorganic liquid, particularly a hydrocarbon. Such hydrocarbons can beparaffinic hydrocarbons containing from 4 to 7 carbon atoms, such asnormal and branched chain butanes, normal and branched chain pentanes,normal and branched chain hexanes and normal and branched chainheptanes. Mixtures of such hydrocarbons also can be employed. Otherhydrocarbons also can be utilized such as cyclohexane, benzene, mixturesthereof, and mixtures of any of the above paraffinic hydrocarbons, orhydrocarbon mixtures with benzene and cyclohexane, or mixtures ofbenzene and cyclohexane.

Although not preferred, immiscible liquids which are more dense thanwater can be employed, such as chloroform, carbontetrachloride, anddichloromonofluoromethane, provided such higher density liquids alsohave a boiling point lower than water.

The immiscible or hydrocarbon liquids can have a boiling range fromabout 60 to about 97° C. at atmospheric pressure.

Preferred immiscible hydrocarbon liquids which can be employed in theprocess and system of the invention are normal and branched chainpentanes, normal and branched chain hexanes, and mixtures thereof.

Fresh water can be recovered according to the invention process andsystem from any aqueous solution containing a non-volatile solute,particularly salt water or sea water in which such solute is sodiumchloride. Examples of other aqueous solutions which can be employed forrecovery of fresh water according to the invention, include brackishwaters.

Thus, the present invention is directed broadly to a process forseparating water from an aqueous solution containing a non-volatilesolute, e.g. sea water, by direct contact heat transfer, which comprisesin each stage of a plurality of interconnected like stages, directlycontacting a hot liquid, e.g hot water, with a water immiscible liquidhaving a boiling point lower than such hot liquid in an evaporator zone,causing such immiscible liquid to boil, contacting the vapor of theimmiscible liquid generated during such boiling with a cooler aqueoussolution of a non-volatile solute in a condenser zone, causingcondensation of such vapor and heating the aqueous solution by transferof latent heat from the condensing vapor, and recycling the condensedimmiscible liquid from the condenser zone to the evaporator zone.

The hot aqueous solution of a non-volatile solute, such as sea water, iswithdrawn from the condenser zone of the first stageevaporator-condenser unit, and is flashed. The vapor so generated iscondensed in direct contact with recycled fresh water withdrawn from theevaporator section of the last evaporator-condenser stage. The resultingrecycled fresh water is thus heated by the condensing vapor, and isreturned to the evaporator section of the first stage unit, after beingfurther heated by an external energy source to supply the necessaryenergy of separation. A portion of the fresh water stream withdrawn fromthe last stage as noted above is removed as product water.

The spent and concentrated aqueous solution of non-volatile solute, e.g.brine, following flashing is removed as a waste stream.

Prior to introduction of the feed of the aqueous solution of anon-volatile solute, or sea water into the condenser section of the laststage evaporator-condenser unit, such solution preferably is passed inheat exchange relation with both the warm fresh water withdrawn from theevaporator section of the last stage unit, and with the warm spentaqueous solution or brine following flashing thereof, for recovery ofany residual heat in such fresh water or spent solution, to warm theaqueous feed solution or sea water.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in greater detail below, taken inconnection with the accompanying drawing wherein:

FIG. 1 is a diagrammatic flow sheet illustrating the process and systemof the invention employing staged evaporator-condenser units;

FIG. 2 illustrates the use of a wire mesh in the evaporator section ofthe stages to promote mixing of the liquid phases;

FIG. 3 illustrates a form of wheel having wire screen paddles positionedin the evaporator section of the stages to promote mixing between theliquid phases;

FIG. 4 is a transverse section through the wheel of FIG. 3, taken online 4--4 of FIG. 3; and

FIG. 5 illustrates introduction of a jet of hot water into theevaporator section to enhance the heat transfer coefficient.

Referring to FIG. 1, hot fresh water, e.g. at a temperature of about212° F., is introduced at 10 into the first stage unit A of a series oflike evaporator-condenser units, shown as three in number, and includingthe two subsequent stage units B and C. It will be understood that thenumber of such stages or evaporator-condenser units can be varied andcan be less than three or more than three, depending on the conditionsof operation. Each of the evaporator-condenser units contains a lowerevaporator section 12 and an upper condenser section 14, separated by anintermediate bubble cap tray 16 between the evaporator and condensersections, the bubble cap tray 16 containing a plurality of bubble capsof the conventional type, one of which is illustrated at 18.

A body of liquid hydrocarbon, n-pentane, for example, indicated at 20,is maintained in the lower evaporator section 12 of each of theevaporator-condenser units.

Introduction of the hot fresh water into the pool of liquid n-pentane 20in the evaporator section of the first stage unit A produces boiling ofthe hydrocarbon, and such boiling is maintained during continuousintroduction of the hot fresh water into the unit A.

For successful operation of the invention process an essential criterionis the provision of conditions which provide intense continuous boilingof the liquid hydrocarbon in each of the stages. The intensity ofboiling depends upon the intensity of mixing between the two phases. TheReynolds number with which the hot water flows into the hydrocarbonphase can be taken as a measure of the extent of such mixing. Theresults of experiments show that for an inlet Reynolds number of 8×10³the magnitude of the volumetric heat transfer coefficient is 67,000 Btu/(hr) (ft³) (°F.) without any special design to promote mixing betweenphases.

However, by appropriate design and incorporation of certain othercomponents and features, intimate mixing between the hot water andhydrocarbon phases to promote intense boiling can be assured. With suchenhanced mixing, the magnitude of the volumetric heat transfercoefficient can be increased several times that of the value notedabove. Thus, as seen in FIG. 2, the use of a wire mesh block 19 in thebody of liquid hydrocarbon in the evaporator section 12 to causeheterogeneous nucleation and mixing of the two phases has been found tobe favorable for promoting boiling.

As illustrated in FIGS. 3 and 4, the use of a wheel 21, particularly onehaving wire screen paddles 23 in the evaporator section 12, whichrotates to promote turbulence and mixing of the two liquid phases whenthe hot water at 10 is introduced into the body of immiscible liquid orhydrocarbon 20 in the evaporator and impinges on the wheel, alsopromotes boiling. Pilot plant tests employing a wheel made of wirescreen placed in the evaporator, as in FIGS. 3 and 4, resulted in avolumetric heat transfer coefficient as high as 156,000Btu/(hr)(ft³)(°F.) with a temperature differential (ΔT) between the hotwater and the hydrocarbon of about 3.5° F. Higher coefficients of heattransfer can also be obtained by introducing the hot water through anozzle 25 in the form of a jet 25' at high velocity into the pool ofhydrocarbon in the evaporator, as illustrated in FIG. 5.

The immiscible hydrocarbon vapor generated during boiling flows throughthe bubble caps 18 and is condensed in contact with a relatively coldlayer of sea water 22 in the condenser section 14 to form a layer ofhydrocarbon liquid 24 which floats on the sea water layer 22. Thecondensation of the hydrocarbon vapor in contact with the sea watercauses the latter to become heated due to transfer of latent heatreleased from the condensing vapor. The thickness of the water layer 22and the rate of flow of the water layer across the condenser should besuch that substantially all of the hydrocarbon vapor bubbling throughthe water layer is condensed therein.

An additional amount of immiscible liquid or hydrocarbon can be addedduring operation for circulation between the evaporator and condenser ofeach stage to ensure that a hydrocarbon layer of sufficient thickness isalways maintained in both chambers, particularly the evaporator. Anyhydrocarbon vapor which escapes condensation in the condenser can becondensed by means of a heat exchanger (not shown). The condensedhydrocarbon, such as n-pentane, at 24 which accumulates at the top ofthe sea water layer 22 in each stage, is transferred back to theevaporator section 12 of the unit through a side tube 26. Although thepressure in the evaporator section 12 is usually slightly higher thanthat in the condenser section 14, the above noted transfer of condensedhydrocarbon liquid from the condenser section to the evaporator sectionthrough tube 26 can be achieved by maintaining a sufficientgravitational head of the hydrocarbon liquid in the condenser.

The water 27 in the bottom of the evaporator section 12 of the firststage A is transferred via line 28 to the evaporator section 12 of thenext stage B and the water 27' from the bottom of the evaporator sectionof stage B, is transferred via line 30 to the evaporator section 12 ofthe last stage C. The temperature of the fresh water passing from thebottom of stage A and progressively to the evaporator sections of stagesB and C, decreases monotonically from the first stage A to the laststage C due to the transfer of heat from the hot water to the boilinghydrocarbon in the evaporator sections of each of the stages. However,the temperature of the hot water entering the evaporator section 12 ofthe first stage A is sufficiently high, and the other conditions ofoperation such as amount and composition of the hydrocarbon in eachstage, and the flow rate of the water from the evaporator section 12 ofthe first stage A to the evaporator section of the last stage C, aresuch that the temperature of the hot water entering the evaporatorsection of the last stage C is still high enough to produce boiling ofthe hydrocarbon therein.

In a similar manner, sea water feed in the condenser section 14 of thelast stage C is transferred from such condenser section via line 32 andpump 34 to the condenser section 14 of the second stage B, and thencefrom the condenser section of stage B, via line 36 and pump 38 to thecondenser section 14 of the first stage A. The temperature of the seawater 22 in the condenser section of each stage increases as the seawater stream flows in the above noted manner from stage C to stages Band A, the sea water leaving the condenser section of stage A having thedesired high temperature due to heat transfer from the hydrocarbon vaporin each of the stages to the sea water therein. Thus, it is seen thatthe hot fresh water proceeds initially to the evaporator section of thefirst stage unit A and then progressively through the evaporatorsections of the successive stages to the last stage unit C, and therelatively cool sea water feed proceeds countercurrently or in theopposite direction to the condenser section of the last stage unit C andthen successively to the condenser sections of the preceding stages andfinally to the first stage unit A.

Since two streams of water (fresh water and sea water) are flowing fromstage to stage in opposing directions, pumps such as 34 and 38 arerequired for at least one of the streams. It is desirable, however, tomaintain pressures in the stages A, B and C at such values as tominimize pumping energy. The pressures in the respective stages can bereadily selected and optimized for a given variation in temperatures bymaintaining the appropriate liquid composition of the hydrocarbon ineach stage. By suitable selection of the composition of the hydrocarbonliquids in each stage, pressure in each of the stages can be madeapproximately equal. Such pressure can range from about 1 to about 5atmospheres absolute. Thus, although pumps 34 and 38 are required fortransferring the sea water from one stage to the preceding stage, thepumping energy required for this purpose is minimal.

Each of the other evaporator-condenser stage units B and C in the systemoperate in substantially the same manner as the first stage A, notedabove, to provide and maintain intense boiling of the liquid hydrocarbonin the evaporator section by direct contact heat transfer from the hotfresh water to the liquid hydrocarbon, passage of the hydrocarbon vaporthrough the bubble caps and into direct contact with the relativelycooler sea water in the condenser section, to condense the hydrocarbonvapor and heat the sea water by transfer of latent heat of vaporcondensation to the sea water, and return of condensed hydrocarbonliquid by gravity through tube 26 to the evaporator section.

The hot sea water is withdrawn from the condenser section of the firststage A and is passed via line 40 into the evaporator section 42 of aflash evaporator 44, which can be in the form of a multi-stage directcontact condenser and flash evaporator of known design, only one stage44 of which is shown. The flash evaporator includes a condenser section46, with a bubble cap tray 48 and bubble caps, one of which isillustrated at 50, separating the evaporator and condenser sections. Thewater vapor generated during flashing of the hot sea water in theevaporator section 42 passes through the bubble caps 50 and is condensedin direct contact with recycled fresh water at 52 in the condensersection. Such recycled fresh water thus becomes heated due to transferof the latent heat released in the condensation of the water vapor. Thecombined stream of condensed water and fresh water at 54 is furtherheated in a heat exchanger 56 to supply the necessary energy ofseparation for the desalination, and the resulting hot fresh water at 10is then introduced into the evaporator section 12 of the first stageunit A, as previously noted. Due to direct contact heat exchange betweenthe phases in the stages A, B and C, boiling and condensation ofimmiscible liquid or hydrocarbon can occur with smaller temperaturedifference therebetween, and the thermal efficiency of the process isenhanced. Consequently, the heat input source illustrated by the heatexchanger 56 can employ low quality heat energy such as relatively lowpressure steam. The heat exhanger 56 can be, for example, a tube-typeheat exchanger wherein the recycled heated fresh water stream is passedthrough tubes in a jacket in countercurrent relation to steam flowingthrough the jacket.

The cooled fresh water at 27" in the bottom of the evaporator section 12of the last stage unit C, and at a temperature, for example, of about120° F., is passed via line 58 through a heat exchanger 60 to transferany residual heat to the incoming sea water feed 62. A portion of theexiting fresh water at 64 is withdrawn as product water at 66, while theremaining portion of the fresh water is recycled via line 68 to thecondenser section 46 of the evaporator-condenser 44 for flashing the hotsea water. The sea water feed 62 exiting the heat exchanger 60 is passedvia line 70 through a second heat exchanger 72 in heat exchange relationwith spent concentrated sea water or brine at 74 withdrawn from theflash evaporator 44. Thus, the residual heat from the flashed spent seawater is recovered by heat transfer to the incoming sea water feed. Theresulting warm sea water feed exiting heat exchanger 72, at atemperature for example of about 110° F., is passed via line 76 into thecondenser section 14 of the last stage evaporator-condenser unit C, andthe spent brine discharged from the condenser 72 is conducted via line78 to waste.

The arrangement of the heat exchangers and the flows of the streams 58,62 and 74 can be varied, since the detailed heat and material balancemay necessitate different arrangements of these flows to attain desiredtemperatures of the streams. Thus, for example, instead of introducingall of the incoming sea water feed 62 into the condenser section 14 ofthe last stage C, a portion of such stream may be required to berejected after passing through heat exchanger 60 and cooling the freshwater stream coming from the evaporator section of the last stage C, inorder to maintain the proper heat balance of the process.

From the foregoing, it is seen that the invention provides a simpleprocess and system of improved performance, reliability, and efficiencyfor producing fresh water from an aqueous solution of a non-volatilesolute, particularly from sea water or brine. The basic principles ofthe invention process and system for heat transfer from one fluid to asecond fluid can also be employed to transfer or transport heat fromgeothermal and ocean brine to other fluids for various applications.

While I have described particular embodiments of my invention forpurposes of illustration, it is understood that other modifications andvariations will occur to those skilled in the art, and the inventionaccordingly is not to be taken as limited except by the scope of theappended claims.

What is claimed is:
 1. A system for separating water from an aqueoussolution containing a non-volatile solute by direct contact heattransfer which comprises:(a) a plurality of unitary evaporator-condenserstages, each stage comprising a lower evaporator chamber, an uppercondenser chamber means for enabling vapor to pass from said lowerevaporator chamber into said upper condenser chamber and means forenabling condensate to pass from said upper condenser chamber into saidlower evaporator chamber; (b) means for introducing hot water into thelower evaporator chamber of a first stage of said evaporator-condenserstages and into contact with an immiscible liquid therein to causeboiling of said immiscible liquid to form a vapor therefrom; (c) firstconduit means, interconnecting the lower evaporator chamber of each ofsaid evaporator-condenser stages, for passing said hot water from theevaporator chamber of said first evaporator-condenser stage to theevaporator chambers of succeeding stages to the evaporator chamber of alast stage of said evaporator-condenser stages, said hot watercontacting an immiscible liquid in each of said lower evaporatorchambers and causing boiling therof to form a vapor therefrom; (d) meansfor introducing an aqueous solution containing a non-volatile soluteinto the upper condenser chamber of the last stage of saidevaporator-condenser stages and into contact with the vapor of theboiled immiscible liquid from the first stage lower evaporator chamberto cause heating of the aqueous solution and condensing of the boiledimmiscible liquid vapor; (e) second conduit means interconnecting theupper condenser chambers of each of said evaporator-condenser stages forpassing said aqueous solution from the upper condenser chamber of thelast evaporator-condenser stage successively into the upper condenserchamber of preceding evaporator-condenser stages to the firstevaporator-condenser stage; (f) a flash chamber; (g) means forwithdrawing heated aqueous solution from the upper condenser chamber ofsaid first evaporator-condenser stage and introducing said withdrawnheated aqueous solutes into the flash chamber for flashing vaportherefrom; (h) means for condensing the flashed water vapor, and, (i)means for withdrawing said condensed water as product.
 2. A system asdefined in claim 1, wherein said means for enabling vapor to pass fromsaid lower evaporator chamber into said upper condenser chamber of eachevaporator-condenser stages comprises at least one bubble cap traydisposed between the upper condenser and lower evaporator chamber ofeach evaporator-condenser stage.
 3. The system as defined in claim 1,including means for pumping said aqueous solution from the uppercondenser chamber of said last evaporator-condenser stage, to the uppercondenser chamber of each preceding evaporator-condenser stage to thefirst evaporator-condenser stage, and said means for enabling condensateto pass from said upper condenser chamber into said lower evaporatorchamber of each evaporator-condenser stage comprises conduit means forenabling said condensate to pass by gravity.
 4. The system as defined inclaim 1, wherein said aqueous solution containing a non-volatile solutecomprises sea water, said immiscible liquid comprises a liquidhydrocarbon, and said system further comprises first indirect heatexchanger means for heating said sea water introduced into the uppercondenser chamber of the last evaporator-condenser stage with hotaqueous water withdrawn from the lower evaporator chamber of the lastevaporator-condenser stage and second indirect heat exchanger means forfurther heating said sea water introduced into the upper condenser ofthe last evaporator-condenser stage with sea water withdrawn from theflash evaporator after flashing thereof.
 5. The system as defined inclaim 1, including means in each lower evaporator chamber to promotemixing of said hot water and said immiscible liquid.
 6. The system asdefined in claim 5, wherein said last mentioned means comprises a wheelhaving wire screen paddles mounted for rotation within each said lowerevaporator chamber, rotation of said wheel being caused by impingementthereon of said hot water introduced in said lower evaporator chamber.7. The system as defined in claim 5, wherein said last mentioned meanscomprises a wire mesh mounted within each lower evaporator chamber. 8.The system as defined in claim 5, wherein said last mentioned meansincludes means for introducing a jet of hot feed water into each lowerevaporator chamber.
 9. A system for separating water from sea water bydirect contact heat exchange, which comprises:(a) a plurality of unitaryevaporator-condenser stages, each stage comprising a lower evaporatorchamber and an upper condenser chamber, bubble cap tray means disposedbetween the evaporator chamber and the condenser chamber of eachevaporator-condenser stage, and pipe means interconnecting the uppercondenser chamber and the lower evaporator chamber in each of saidevaporator-condenser stages for passing of liquid by gravity from theupper condenser chamber to the lower evaporator chamber. (b) a firstconduit interconnecting the lower evaporator chamber in each of saidevaporator-condenser stages to the upper condenser chambers of each ofthe preceding evaporator-condenser stages, (c) a second conduitinterconnecting the upper condenser chambers of each of saidevaporator-condenser stages, (d) pump means in said second conduit forpumping liquid from the upper condenser chamber of a lastevaporator-condenser stage to the upper condenser chambers of each ofthe preceding evaporator-condenser stages, (e) a flash evaporator, saidflash evaporator including an evaporator section and a condensersection, (f) a third conduit for passage of hot sea water from the uppercondenser chamber of a first evaporator-condensate stage to theevaporator section of said flash evaporator, (g) a fourth conduit forpassage of fresh water from the condenser section of said flashevaporator to the lower evaporator chamber of said firstevaporator-condenser stage for direct contact heat exchange with animmiscible liquid therein, (h) a first indirect heat exchanger in saidlast mentioned conduit for heating said fresh water, (i) a fifth conduitinterconnecting the evaporator chamber of the last evaporator-condenserstage with the condenser section of said flash evaporator for passage ofwater from said last mentioned lower evaporator chamber to saidcondenser section, (j) a product conduit connected to said fifth conduitfor withdrawing product water, (k) a second indirect heat exchanger, (l)a sixth conduit for introduction of sea water feed into the uppercondenser chamber of said last evaporator-condenser stage, said fifthand sixth conduits passing through said second indirect heat exchangerfor transfer of heat from said water to said sea water feed for heatingsame,(m) a third indirect heat exchanger, (n) a seventh conduit forwithdrawing spent sea water from the evaporator section of said flashevaporator, said sixth and seventh conduits passing through said thirdindirect heat exchanger for transferring heat from said spent sea waterto said sea water feed for further heating same prior to introductioninto the condenser chamber of said first evaporator-condenser stage, and(o) conduit means for withdrawing spent sea water waste from said thirdindirect heat exchanger.